Automatic Power Transmission

“Automatic Power Transmission”- By Sumukh T Deshpande & Harshvardhan P Patil

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AutomaticPower

Transmission -a gearshift in technology

AuthorsSumukh T Deshpande & Harshvardhan P Patil

205, Aekdanth Apartments, c/o K.P.Harlapur,Nehru Road, Rajaram Marg, 4th Cross,Near Lele Ground, Tilakwadi, Chidambar-Nagar,Belgaum. Belgaum.Ph: - 09886574634 Ph: - 09448634898

Email Id - [email protected] Email Id: [email protected]

4th Semester

Department of Mechanical Engineering

Gogte Institute of Technology

Udyambag, BELGAUM -590008.

Abstract‘VARIANTS’ are the flavour of the day. They catch the customer's fancy until a newer version beguiles


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him. This has led to continuous improvement in car technology. Automatic Power transmission is also one of the

outcomes of this continuous improvement. The automatic transmission's primary job is to allow the engine to

operate in its narrow range of speeds while providing a wide range of output speeds.

The automatic transmission is a kind of ‘no-muss, no-fuss’ form of gear shifting. Automatic

transmission means - No more missed shift gates, no engine lugging or racing, and no torn-stocking, high-heel

clutch-pedaling dramas. The shift lever is moved to get the car in motion and then left alone until the car is

stopped. No clutch pedal to push or shifter to change between three, four, or even five forward gears makes for

a minimum of driver effort. Both the automatic transmission (with its torque converter) and a manual

transmission (with its clutch) accomplish exactly the same thing, but they do it in totally different ways. In this

technical paper on ‘Automatic Power Transmission – a gearshift in technology’ we first introduce what really is

automatic transmission, then we discuss different transmission components (in brief), shifting mechanism of

gears, compare it with manual transmission, before concluding.

Keywords: - Transmission System, Planetary Gear Sets, Torque Converter, Clutches and Bands,

Governor, Gear Lever.

INDEX


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1. Introduction 4

2. Transmission Components 4

1. Planetary Gear Sets 5

2. Torque Converter 6

3. Hydraulic System 8

4. Oil Pump 9

5. Clutch 10

6. Clutch Packs 10

7. Bands 11

8. Governor, Vacuum Modulation, Throttle Cable 11

3. Electronic Controls 13

4. Computer Controls 14

5. Shifting Mechanism Of Gears 15

6. Latest Trends In Technology 16

7. Automatic v/s Manual Transmission 17

8. Conclusion 18

References 19

Abstract Word Count: - 210

Paper Word Count: - 3515


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1. Introduction

The transmission is a device that is connected to the engine and sends the power from

the engine to the drive wheels. An automobile engine runs at its best at a certain RPM

(Revolutions Per Minute) range and it is the transmission’s job to make sure that the power is

delivered to the wheels while keeping the engine within that range.

The earliest automobiles offered only manual transmissions. But as traffic got worse,

engineers began searching for a way to have the car “automatically” shift from one gear to

another leading to evolution of automatic transmission system. An automatic transmission is

a gearbox that can change gear ratios automatically as the car moves, thus freeing the driver

from having to shift gears manually. These cars are provided with only two-foot pedals, the

accelerator and the brake pedal. They do not have a gear lever either. Now let’s drive our way

through the paper to discover how this is possible.

2. Transmission ComponentsThe modern automatic transmission consists of many components and systems that are

designed to work together in a symphony of clever mechanical, hydraulic and electrical

technology that has evolved over the years into what many mechanically inclined individuals

consider being an art form [5]. The main components that make up automatic transmissions

are: -

Planetary Gear Sets.

The Torque Converter.

The Hydraulic System.

The Governor, Modulator or Throttle Cable.


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2.1 Planetary Gear Sets

A manual transmission locks and unlocks different sets of gears to the output shaft to

achieve the various gear ratios, while in an automatic transmission; the same set of gears

produces all the different gear ratios. The gears are never physically moved and are always

engaged to the same gears. This is accomplished through the use of planetary gear sets.

The basic planetary gear set consists of a sun gear, a ring gear and two or more planet

gears, all remaining in constant mesh (Figure 1.) The planet gears are connected to each other

through a common carrier, which allows the gears to spin on shafts called “pinions” which

are attached to the carrier [1].


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The input shaft is connected to the ring gear. The Output shaft is connected to the planet

carrier that is also connected to a “Multi-disk” clutch pack. The sun gear is connected to a

drum, which is also connected to the other half of the clutch pack. Surrounding the outside of

the drum is a band that can be tightened around the drum when required to prevent the drum

with the attached sun gear from turning.

Different combinations are possible using two or more planetary sets connected in various

ways to provide the different forward speeds and reverse that are found in modern automatic


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transmissions.

2.2 Torque Converter

To transfer motion from engine to the transmission is the work of Torque Converter.

This unit does what its name implies; it converts or “multiplies” engine torque at low speeds

and couples engine to the drive train hydraulically at higher speeds. On automatic

transmissions, the torque converter takes the place of the clutch found on standard shift

vehicles.

A torque converter (Figure 3.) is a large doughnut shaped device that is mounted

between engine and transmission. It consists of three elements necessary for proper

transmission of motion: -

The Pump: - The pump is mounted directly to the converter housing that in turn is

bolted directly to engine’s crankshaft and turns at engine speed.

The Turbine: - The turbine is inside the housing and is connected directly to the

input shaft of the transmission providing power to move the vehicle. The Impeller and

Turbine transfer fluid between each other to drive the Input Shaft of the transmission.

The Stator: - The stator is mounted to a one-way clutch so that it can spin freely in


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one direction but not in the other. The Stator is a device used in the converter to

multiply torque by giving proper direction to the fluid flowing between the impeller

and turbine [1].

The overall purpose of torque converter is to: -

Provide a coupling between the engine and transmission.

Supply a method of driving the transmission oil pump.

Provide a unit that incorporates a flywheel for starting.

Supply a multiplication of engine torque of approximately

2.2:1 at low speeds and 1:1 at higher speeds [7].

The principle behind a torque converter is like taking a fan that is plugged into wall

and blowing air into another fan that is unplugged. (Figure 3.1.) The difference with a torque

converter is that instead of using air, it uses oil or transmission fluid, to be more precise [3].

The impeller is on the engine side, the turbine on the tranny side. Both of these

components have blades that catch the transmission fluid and cause them to spin. (Figure 3.)


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As one fan begins to spin, the other spins as well. Through centrifugal force, the fluid moves

to the outside of the blades, where it is redirected by a third fan, called a stator, back to

turbine side. This continual flow of fluid is what causes the power to be multiplied.

Each of the three elements has fins mounted in them to precisely direct the flow of oil

through the converter. With the engine running, transmission fluid is pulled into the pump

section and is pushed outward by centrifugal force until it reaches the turbine section. The

fluid continues in a circular motion back towards the center of the turbine where it enters the

stator. If the turbine is moving considerably slower than the pump, the fluid will make contact

with the front of the stator fins, which push the stator into the one way clutch and prevent it

from turning. With the stator stopped, the fluid is directed by the stator fins to re-enter the

pump at ‘helping’ angle providing a torque increase.

As speed of turbine catches up with pump, the fluid starts hitting the stator blades on

the backside causing the stator to turn in the same direction as the pump and turbine. As the

speed increases, all three elements begin to turn at approximately the same speed. The torque

converter offers the advantage of multiplying the turning power provided by the engine. It is

connected to the motor by means of a metal rod known as the transmission shaft (called the

input shaft), which fits next to but does not touch the engine crankshaft [3].

2.3 Hydraulic System


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The Hydraulic system (Figure 4.) sends transmission fluid under pressure to all parts

of the transmission and torque converter. The newer hydraulic systems are complex and are

combined with computerized electrical components. Transmission fluid serves a number of

purposes including: shift control, general lubrication and transmission cooling. Unlike the

engine, which uses oil primarily for lubrication, every aspect of a transmission’s functions is

dependent on a constant supply of fluid under pressure. In fact, most of the components of a

transmission are constantly submerged in fluid including the clutch packs and bands. The

friction surfaces on these parts are designed to operate properly only when they are

submerged in oil [2].

The passageways shown in the adjacent figure, route the fluid to all the different

components in the transmission. Passageways molded into the metal are an efficient way to


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route fluid.

2.4 Oil Pump

The transmission oil pump (Figure 5.) is responsible for producing all the oil

pressure that is required in the transmission. The oil pump is mounted to the front of the

transmission case and is directly connected to a flange on the torque converter housing. Since

the torque converter housing is directly connected to the engine crankshaft, the pump will

produce pressure whenever the engine is running as long as there is a sufficient amount of

transmission fluid available.

2.5 Clutches

Clutch has setup of a series of friction discs placed between steel plates. (Figure 6.)

The clutch also has a piston and return springs. This whole assembly is known as a clutch

pack. When fluid pressure is applied to the clutch pack, the piston engages, locking the

assembly together and driving the wheels.


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When pressure releases (for instance, the engine goes to idle), the piston disengages

and the wheels stop turning. The planet carrier gets connected to the torque converter housing

by a clutch. The small sun gets disconnected from the turbine by a clutch so that it can

freewheel [1].

2.6 Clutch Packs


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A clutch pack (Figure 7.) consists of alternating disks that fit inside a clutch drum.

Half of the disks are steel and have splines that fit into groves on the inside of the drum. The

other half have a friction material bonded to their surface and have splines on the inside edge

that fit groves on the outer surface of the adjoining hub.

There is a piston inside the drum that is activated by oil pressure at the appropriate

time to squeeze the clutch pack together so that the two components become locked and turn

as one [1].

2.7 BandsA band is a steel strap with friction material bonded to the inside surface. One end of

the band is anchored against the transmission case while the other end is connected to a

servo. At the appropriate time hydraulic oil is sent to the servo under pressure to tighten the

band around the drum to stop the drum from turning.


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The bands in a transmission are, literally, steel bands that wrap around sections of the

gear train and connect to the housing. (Figure 8.) The metal rod is connected to the piston,

which actuates the band. Hydraulic pressure, routed into the cylinder by a set of valves,

causes the pistons to push on the bands, locking that part of the gear train to the housing.

2.8 Governor, Vacuum Modulation, Throttle Cable

The Governor is a clever valve that tells the transmission how fast the car is going.

(Figure 9.) It is connected to the output, so the faster the car moves, the faster the governor

spins. Inside the governor is a spring-loaded valve that opens in proportion to how fast the


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governor is spinning.

The faster the car goes, the more the governor valve opens and the higher the pressure

of the fluid it lets through. Vehicle speed is not the only thing that controls when a

transmission should shift, the load that the engine is under is also important.

There are two types of devices that serve the purpose of monitoring the engine load: -

The Throttle Cable simply monitors the position of the gas pedal through a cable that

runs from the gas pedal to the throttle valve in the valve body.

The Vacuum Modulator monitors engine vacuum by a rubber vacuum hose that is

connected to the engine. . The modulator is attached to the outside of the transmission

case and has a shaft that passes through the case and attaches to the throttle valve in

the valve body. When an engine is under a light load or no load, high vacuum acts on

the modulator that moves the throttle valve in one direction to allow the transmission

to shift early and soft. As the engine load increases, vacuum is diminished which

moves the valve in the other direction causing the transmission to shift later and more

firmly.

These three components are important in the non-computerized transmissions.

They provide the inputs that tell the transmission when to shift. As the weights pull

further out against the springs, more oil pressure is allowed past the governor to act on the

shift valves that are in the valve body which then signal the appropriate shifts.


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3. Electronic Controls

Electronically controlled transmissions, which appear in some newer cars, still use

hydraulics to actuate the clutches and bands, but each hydraulic circuit is controlled by an

electric solenoid. This simplifies the plumbing on the transmission and allows for more

advanced control schemes.

In addition to monitoring vehicle speed and throttle position, the transmission

controller can monitor the engine speed. Using this information and an advanced control

strategy based on fuzzy logic -- a method of programming control systems using human-type


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reasoning - electronically controlled transmissions can do things like: -

1. Downshift automatically when going downhill to control speed and reduce wear on

the brakes.

2. Upshift when braking on a slippery surface to reduce the braking torque applied by the

engine.

3. Inhibit the upshift when going into a turn on a winding road.

4. While driving on an uphill, winding mountain road inhibition of upshift is crucial. In

straight sections of the road, the transmission shifts into second gear to give enough

acceleration and hill-climbing power.

5. When a curve is encountered one tends to taking the foot off the acceleration pedal

and possibly applying the brake. Most transmissions will upshift to third gear, or even

overdrive, when this is done [1].

Then when the car accelerates out of the curve, they will downshift again. But

in a manual transmission car, one would probably leave the car in the same gear the

whole time. Some automatic transmissions with advanced control systems can detect

this situation after the car has gone around a couple of the curves, and "learn" not to

upshift again.

4. Computer Controls

The computer uses sensors on the engine and transmission to detect such things as

throttle position, vehicle speed, engine speed, engine load, stop light switch position, etc. to

control exact shift points as well as how soft or firm the shift should be. Some computerized

transmissions even learn your driving style and constantly adapt to it so that every shift is

timed precisely when you would need it. The computer monitors and makes sure that the


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driver does not select a gear that could over speed the engine and damage it. Another

advantage to these "smart" transmissions is that they have a self-diagnostic mode, which can

detect a problem early on and warn you with an indicator light on the dash [7].

5. Shifting Mechanism Of Gears

The lever-type selector features R, N, D, 4, 3, 2 and 1 positions, selected by pressing a release

button and moving the lever to the desired position.

R (reverse): vehicle must be fully stopped before selecting this position.

N (neutral): for starting engine, for long idling periods and for parking.

2–5 or D: for normal driving conditions.

2–3/2–4: low range for operating under load.

2: for snow-covered roads or steep climb.

1: for very slow driving off-highway (rough terrain).

As described earlier, planetary gears are used in automatic transmission. But how actually

the gears are shifted, has been discussed in this section: -

1. Connecting the ring gear to the input shaft coming from the engine, and connecting

the planet carrier to the output shaft, and locking the sun gear so that it can't move. In

this scenario, when the ring gear turns, the planets will "walk" along the sun gear

(which is held stationary) causing the planet carrier to turn the output shaft in the same

direction as the input shaft but at a slower speed causing gear reduction. (similar to a

car in first gear).

2. If the sun gear is unlocked and any two elements are locked together, this will cause

all three elements to turn at the same speed so that the output shaft will turn at the

same rate of speed as the input shaft. This is like a car that is in third or higher gear.


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3. If the planet carrier is locked from moving and power is applied to the ring gear, this

will cause the sun gear to turn in the opposite direction giving us reverse gear. The

clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing

both to turn at the same speed.

4. If both clutch pack and band were released, the system would be in neutral. Turning

the input shaft would turn the planet gears against the sun gear, but since nothing is

holding the sun gear, it will just spin free and have no effect on the output shaft.

5. An overdrive has a faster output speed than input speed. In order to improve

efficiency, cars have a mechanism that locks up the torque converter so that the output

of the engine goes straight to the transmission. In this transmission, when overdrive is

engaged, a shaft that is attached to the housing of the torque converter (which is

bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The

small sun gear freewheels, and the overdrive band holds the larger sun gear. Now the

planet carrier is for input, the sun gear fixed and the ring gear for output. So the output

spins once for every two-thirds of a rotation of the engine. This allows cars to drive at

freeway speed while the engine speed stays nice and slow [4].

The automatic transmission not only transmits power intelligently, but also performs

following functions: -

If the car is in overdrive, the transmission will automatically select the gear based on

vehicle speed and throttle pedal position.

If the vehicle is accelerated gently, shifts will occur at lower speeds than if accelerated

at full throttle.

If the shift selector is moved to a lower gear, the transmission will downshift unless

the car is going too fast for that gear. If car is going too fast, it will wait until the car slows


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down and then downshift.

If the transmission is put in second gear, it will never downshift or upshift out of

second, even from a complete stop, unless the shift lever is moved.

6. Latest Developments in Automatic Power Transmission

Continuously Variable Transmission (CVT): - It is a different type of automatic

transmission, which can smoothly alter its gear ratio by varying the diameter of a pair of

belt or chain-linked pulleys, wheels or cones. CVT designs are as fuel-efficient as manual

transmissions in city driving, worse in highway driving, and are more expensive to buy

and maintain. The expense reflects the difficulty of manufacturing a reliable product. [6]

The Lockup Torque Converter: - To increase efficiency and fuel mileage, most modern

automatic transmissions also have something called a lockup torque converter. As the

speed of the car nears 65 kilometer per hour, the highly pressurized transmission fluid is

channeled through transmission shaft and activates a clutch piston. This metal pin locks

the turbine to the impeller, in effect bypassing the torque converter. It remains this way

until the vehicle slows below 65 kph, at which point the clutch piston disengages and the

torque converter kicks in again.

Automatic Transmission Manual Transmission

1. Gear system Same set of gears produces all of

the different gear ratios.

Locks and unlocks different sets of

gears to the output shaft to achieve

the various gear ratios.

2. Clutch pad and

a gearshift lever

Not required. Necessary.


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3. Number of

clutches

Automatics have clutch packs. Only one.

4. Fuel Efficieny More/Same. Less.

5. Cost Expensive than manual (5 % more) Cheaper than Automatic.

6. Active cooling Required. Not required.

7. When the

battery is dead

Car cannot be push started. Cars with this transmissions can

often be started by pushing the car

into motion and then engaging the

clutch in third or second gear. (push

start)

8. Driver

interaction

Less. More.

9. Learning to

drive

Easy. Difficult.

10. Gear shifting Smooth and quick. Driver dependant.

7.Comparison: -

Table 1. Comparison Between Automatic and Manual Transmission

8. Conclusion

Automatic transmission technology is more widely accepted. While it is true that it

cannot be adopted for all types of transport, the carriers’ survey had no negative performance

aspects to report. In fact, long-term tests under normal operating conditions have shown that

this type of gearbox provides either increased performance or at least the same performance

as the manual transmissions. Carriers noted only two major disadvantages of automatic


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transmissions, compared to manual higher weight and a higher initial cost.

With the absence of the gearshift and the clutch, automatics offer stress-free driving to

discerning customers who have to constantly change gears in the stop-and-go type traffic.

Discussions with manufacturers of semi-automatic and automatic transmissions indicate that

these gearboxes are gaining in popularity.

One reason for the adoption of automatic transmission is the scarcity of professional

drivers. This new technology is easier to use than conventional manual gearboxes, especially

if a majority of a fleet’s runs are on roads that are relatively level or have few steep rises. The

second reason is the potential gain in energy efficiency. Results indicate a substantial

increment in efficiency.

Although such results are promising, they are not enough to conclude that automatic

transmissions are perfect. Continuous improvements are taking place for making car more

‘Auto-motive’. In coming years ‘Automatic Power Transmission’ can prove to be a major

factor that will make driving a more luxurious and stress free.

References

[1] Charles Ofria, “A Short Course on Automatic Transmissions”

[2] Dr.Kirpal Singh, 5th edition-1985,“Automobile Engineering”, Standard Publishers

Distributors

[3] R.B.Gupta, 6th edition- 2004, “Automobile Engineering”, Satyaprakashan New

Delhi.


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[4] R.S.Khurmi, J.K.Gupta, 2003, “Theory Of Machines”, Eurasia Publishing House

PVT .LTD.

[5] “Business Line” -Financial Daily from THE HINDU group of publications, Sunday,

22nd September 2002

[6] www.howstuffworks.com

[7] www.en.wikipedia.org/wiki/Automatic_transmission

Automatic Power Transmission

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